/31 subnet
Mask 255.255.255.254 · 2 usable hosts · 2 addresses
| Subnet mask | 255.255.255.254 |
|---|---|
| Wildcard mask | 0.0.0.1 |
| Binary mask | 11111111.11111111.11111111.11111110 |
| Network / host bits | 31 / 1 |
| Total addresses | 2 |
| Usable hosts | 2 |
| Per /8 · /16 · /24 | 8,388,608 · 32,768 · 128 |
| Example | 192.168.10.77/31 → network 192.168.10.76, range 192.168.10.76 – 192.168.10.77 |
Where /31 is used
Point-to-point links per RFC 3021, saving two addresses.
Block boundaries: a /31 must start at a multiple of 2 addresses, so in the last octet networks start at 0, 2, 4, 6, 8, 10, 12, 14…. Calculate any specific address in the subnet calculator.
Neighbouring prefixes
Questions
How many hosts in a /31?
2 addresses; both usable on point-to-point links (RFC 3021).
What is the subnet mask for /31?
255.255.255.254. In binary: 11111111.11111111.11111111.11111110. The wildcard (inverse) mask is 0.0.0.1.